In joint surgery it is common practice today to anchor components of replacement joints by using as bone cement a two-component resin which polymerizes during the operation at normal temperatures and which, on account of its plastic properties leads to an interlocking of the prosthesis component in the bony sheath. Because of its physical properties, the bone cement shrinks onto the prosthesis resulting in a closed metal-to-cement contact.
The bone cements commonly used are polymethylmethacrylate (PMMA) consisting of powdery bead polymers which are superficially dissolved by liquid monomers and embedded during the polymerization process. During mixing the polymer is immersed in the monomers. The PMMA beads are superficially dissolved and embedded in a composite manner. The processes during polymerization are explained in "Forschung und Fortbildung in der Chirurgie des Bewegungsapparates 1, zur Technik der Zementverankerung", K. Draenert, Art and Science Munich, 1983.
Such a composite structure can be compared with concrete, where air bubbles are likewise included during mixing. Moreover, when the monomer immerses the PMMA beads, filling defects remain. These defects are termed "lee phenomena". Furthermore, in the case of bone cements, the monomer liquid evaporates during the exothermic polymerization, whereby further bubbles are formed. The bubbles formed as mentioned above constitute the major part of the gas enclosures in bone cements.
The chemical reaction of the above-mentioned bone cements is initiated by a starter reaction, wherein, as a rule, dibenzoyl peroxide is activated by an activator such as p-aminotoluidine and then the radical chain polymerization is started. This polymerization proceeds exothermically. The monomer itself is stabilized by hydroquinone. Some bone cements are further stabilized by chlorophyll with simultaneous coloring. The storability of the monomer liquid can also be stalibized by vitamin C.
As a rule, the polymer powder or prepolymer is added to the monomer and mixed in a bowl using a spatula. In the processing phase following the mixing phase the bone cement is applied to the femoral medullary canal or to the bony acetabulum which are both prepared to anchor the cemented prosthesis components; the application is normally performed by hand and sometimes using a syringe. Such a syringe is described in DE-A-28 01 706, inter alia.
Using a syringe, the cement anchorage in the bone can be markedly improved. Therefore, so-called "cement compactors" have been proposed, the principle of which is to impact the cement in the plugged medullary canal to provide transverse anchorage. Such a cement compactor is described in "Journal of Bone and Joint", Vol. 65A, No. 9, December 83, pages 1335-1338.
Moreover, in order to prevent the evaporating monomer from producing unpleasant odors, containers have been proposed for storing and mixing the powder and liquid components, see for instance DE-A-28 01 706. Such containers constitute a closed system, but in practice the two components are all poorly mixed in comparison to cements prepared in the conventional manner with a bowl using a spatula.
Other proposals concern the problem of mixing the cement components and aim at improving the mechanical strength of the bone cement by improved mixing. Such a mixing vessel is described in DE-A-17 66 334. Said vessel has a ball separating the vessel into two chambers; this ball can be pressed through to move freely as a stirring means in one of the two chambers. While this principle may be applied in dental amalgams, it is not suitable for mixing bone cements, as the ball cannot be removed from the curing bone cement and the moving ball results in laminations of the increasingly highly viscous bone cements. Therefore, with this principle it is not possible to achieve uniformly mixed bone cements.
With cement syringes, the filling of the bone bedding is performed in different ways. On the one hand, filling is done from above in the downward direction, as described in the above-mentioned article in the "Journal of Bone and Joint", on the other hand, it is also performed in the other direction, i.e. upwardly from below, by means of a long nozzle, see DE-A-28 14 353.
While it is possible to build up some pressure in the cement by means of a nozzle as for instance described in DE-A-28 14 353, this is not so when applied to the bone, as the resistance of the nozzle reduces the pressure almost completely. With such a nozzle it is not possible to control the pressure during application. Another disadvantage of the nozzle is the so-called "tooth paste phenomenon", i.e. the emerging bone cement piles up like sausages, resulting in critical air, blood and marrow inclusions and laminations which very severely affect the mechanical strength of the applied cement.
All these proposals ultimately aimed at deep anchorage of the endoprostheses component in the bone and at thus improving the long term results of endoprosthesis replacement operations. While with the use of cement syringes the anchoring of the cemented prosthesis can be markedly improved, the mechanical strength of the bone cements thus achieved is still unsatisfactory as compared to bone cement samples prepared in laboratories.
It has been found that the "two-component" synthetic materials used as bone cement incorporate large and small bubbles which as loci minoris resistentiae constitute rated break points of the cement sheath surrounding the endoprosthesis. DE-A-28 14 353, has already disclosed attempts to compress the bone cements by means of appropriate devices during mixing in order to diminish the volume of enclosed air and to thus improve the mechanical strength of the bone cements. With the commercial syringes which in part comprise a one-way cartridge and a simple manually operated piston, it is also possible to render the bone cement somewhat more compact. The piston must not tightly seal the cylinder holding the bone cement in order that the air above the cement is not pressed into the cement. However, none of these known syringes fulfills this requirement, nor does any of these offer the possibility of closing the cement container in such a way that the cement can be exposed to high pressures. Where the ends of these syringes are sealable, these syringes all present the problem of enclosing air at both ends. On account of the laws of laminar flow, the air above the cement is centrally entrained in the cement mass and weakens the very part of the cement sheath that is later to enclose the metal prosthesis. It has been found that the enclosed air is by no means forced through the nozzle, but owing to the laminar flow is forced back laterally into the cement composition resulting in rated break points of the cement sheath.
This also applies to the process of preparing implant materials as described in EP-A-80 101 583, wherein the component mixture is somewhat compressed in a container that is not tightly sealable. Moreover, the pressure generated in this process cannot be adjusted and controlled precisely enough, nor is there sufficient time for handling and building up the pressure during application, in particular because curing proceeds rapidly. Nor does the piston described in DE-U 78 19 584, which fits tightly with the cylinder allow, the air above the cement to escape. Moreover, while the viscous bone cement can be forced out of the cylinder by the piston according to DE-U 78 19 584, the bone cement cannot be highly compressed with this piston. In addition, the lower end of the cylinder is open.
All of the syringes described have the disadvantage of not allowing pressures to build up in a short time and above all of not allowing them to be kept constant over a specific time and to be released quickly. In addition to not allowing high pressures to build up, all the known syringes which are manually compressed cause the muscles to fatigue quickly with the result that constant pressure is not ensured.
The injection piston according to DE-U 78 19 584, like the injection syringe according to DE-A-28 14 353, is at best suited for lamination-free injection of the cement into the bone, but not for effectively prepressurizing the cement to effectively suppress the afore-mentioned bubble formation. In most cases, the excess pressures achievable with the known devices operated manually or with support are by far less than 1 bar (about 100 kPa) and can at best be used to compress the large enclosed air bubbles produced during mixing. This, however, is not sufficient to substantially increase the mechanical strength of the bone cement.
Another problem originates from the fact that the long term success of the operation does not solely depend on the maximum mechanical strength of the bone cement. It is advantageous if the bone cement is somewhat porous at the interface to the bone. The porous surface of the cement enlarges substantially the contact area and histological findings have shown that bony ingrowth will occur as a function of the surface enlargement.
Pore formation can be largely controlled by different additives to the bone cements. However, experience has shown that the stability of the cements decreases with increasing porosity.
This means that any fillers diminish the mechanical strength of the cements resulting in a weakening of these cements if this is not compensated for by bone substance growing into the cement.
The solid (metal) endoprostheses used today place high demands on the cement sheath; it must prevent body liquid and granulating tissue from penetrating into the interface. On the other hand, as explained above, it has been found that bone material will grow from the bone-to-cement interface into the existing pores of the implant under favourable circumstances.
Moreover, it has been found that in conventional bone cements, the liquid monomer flows away quickly or evaporates at the interface to the blood-supplied bone, with the result that the polymerization of the polymer/monomer combination is disturbed and the bone cement especially weak at this point. Furthermore, the bone-to-cement interface is endangered by the fact that in this area, where there are no blood vessels, germs encounter especially favourable conditions for development at the cement surface. Moreover, it is necessary to reduce the polymerization temperature of the cement during surgery by adding fillers in order to prevent the bone from burning. The incorporation of blood and blood coagels results in laminated bone cements and in the formation of rated break points of the implant's cement sheath. By suitable fillers in this outer layer it is possible to achieve both haemostasis and infection prophylaxis as well as effective heat reduction. It is, however, a prerequisite that the mechanical strength of the metal implant's cement sheath is not adversely affected. For this reason, in full shaft implants, resorbable substances such as the tricalcium phosphate described in DE-A-29 05 878 are only suitable in the outer layer facing the bone. The same applies to additives such as the so-called "bone morphogenetic protein". The purpose of this invention is therefore to apply bone cement in such a way that the cement sheath around the metal prosthesis is as homogeneous and mechanically stable as possible and that the porosity of the surface facing the bone is determined by fillers.